(1) Field of the Invention
The present invention relates to a membrane-catalyst layer assembly with reinforcing films, a membrane electrode assembly with reinforcing films, and polymer electrolyte fuel cells.
(2) Description of the Related Art
Fuel cells include electrodes on both surfaces of an electrolyte, and generate power utilizing the electrochemical reaction between hydrogen and oxygen, thus producing only water during the generation of power. Unlike conventional internal combustion engines, fuel cells do not emit environmentally harmful gases such as carbon dioxide and the like, and are therefore expected to prevail as next-generation clean energy systems. Among these fuel cells, polymer electrolyte fuel cells, in particular, have low operating temperatures and low electrolyte resistance, and provide high power while being small because they use highly activated catalysts. For this reason, polymer electrolyte fuel cells are expected to be in practical use soon as cogeneration systems for household use and the like.
These polymer electrolyte fuel cells include a proton-conducting solid polymer electrolyte membrane, and a catalyst layer and a gas diffusion layer, which are smaller in size than the electrolyte membrane, formed in this order on each surface of the electrolyte membrane. In addition, gaskets are located on the electrolyte membrane so as to surround the electrodes each having the catalyst layer and gas diffusion layer, and the electrodes are sandwiched between separators (see, for example, FIG. 2 of Japanese Patent No. 3,052,536). In view of the accuracy of positioning, the gaskets are located to surround the outside of the electrodes, resulting in a gap being formed between each gasket and electrode. The portions of the electrolyte membrane corresponding to these gap portions are constrained by neither the electrodes nor gaskets. Thus, when the polymer electrolyte fuel cells repeatedly generate or do not generate power to subject the electrolyte membrane to repeated moist and dry states, the portions of the electrolyte membrane corresponding to the gap portions experience repeated expansion and shrinkage. As a result, the electrolyte membrane may become worn, and ultimately be broken.
In order to overcome this problem, the polymer electrolyte fuel cell disclosed in, for example, Japanese Patent No. 3,052,536 further includes frame-shaped reinforcing films. These reinforcing films are bonded to the above-described gap portions of the electrolyte membrane. An outer edge portion of each reinforcing film is sandwiched between a gasket and an electrolyte membrane, and an inner edge portion of each reinforcing film is sandwiched between a separator and a gas diffusion layer. In this manner, the polymer electrolyte fuel cell of Japanese Patent No. 3,052,536 uses the reinforcing films to confine the gap portions between the gaskets and the electrodes, thereby preventing expansion and shrinkage of the electrolyte membrane.
In some cases, reinforcing films are provided for a reason different from that described above. In polymer electrolyte fuel cells without the above-described reinforcing films, gaskets are located on outer edge portions of an electrolyte membrane. However, the outer edge portions of the electrolyte membrane provided with the gaskets do not contribute to the generation of power, thus preventing effective use of the electrolyte membrane.
In order to overcome this problem, the polymer electrolyte fuel cell disclosed in FIG. 1 of Japanese Unexamined Patent Publication No. 2004-47230, for example, includes an electrolyte membrane, a catalyst layer formed on the entirety of each surface of the electrolyte membrane, and a gas diffusion layer disposed on each catalyst layer. To provide gaskets, separate reinforcing films are formed to extend outward from the outer edges of the membrane electrode assembly. The formation of separate reinforcing films and the location of the gaskets on the reinforcing films eliminate the portions of the electrolyte membrane that do not contribute to the generation of power.
However, the above-described reinforcing films of the polymer electrolyte fuel cells do not provide sufficient gas barrier properties, because they are formed of a single layer of a polyolefin resin such as polypropylene resin, polyethylene resin or the like. For this reason, a fuel gas supplied to the anode side may leak to the cathode side via the reinforcing film, or an oxidant gas supplied to the cathode side may leak to the anode side via the reinforcing film.
Accordingly, an object of the invention is to provide a membrane-catalyst layer assembly with reinforcing films, a membrane electrode assembly with reinforcing films, and polymer electrolyte fuel cells, which exhibit sufficient gas barrier properties.